Isolation of the plasma membrane from the thyroid

Isolation of 125I-concanavalin A-labeled plasma membrane from unfertilized mouse eggs

A procedure was developed for isolation of plasma membrane (PM) preparations from unfertilized mouse eggs. Zona-free mouse eggs prepared by the method of Boldt and Wolf (Gamete Res 13:213-222, 1986) were labeled with 125I-concanavalin A (ConA) prior to sonication and fractionation on iso-osmotic self-generated Percoll density gradients. Experiments using the ConA-specific sugar alpha-methylmannoside (alpha MM) indicated that 125I-ConA bound specifically to the egg PM. Greater than 95% of 125I-ConA binding to zona-free eggs was blocked in the presence of 0.1 M alpha MM, and incubation of eggs in alpha MM after 125I-ConA labeling caused release of 85-90% of bound label. Fractionation of 125I-ConA-labeled eggs by Percoll density gradient centrifugation yielded a single radioactive peak at density = 1.025, corresponding to egg PM material. Prolonged incubation of 125I-ConA-labeled eggs or egg sonicates prior to fractionation did not alter the location of the radioactive peak, indicating that 125I-ConA did not label other organelles. As a control, human erythrocytes were labeled with 125I-ConA and fractionated under identical experimental conditions and yielded a single radioactive peak at density (1.020) comparable to that observed for 125I-ConA-labeled eggs. These results indicate that 125I-ConA can be used as a specific marker to support PM isolation from small numbers of zona-free mouse eggs.

Adenosine 3',5'-monophosphate modulates thyrotropin receptor clustering and thyrotropin activity in culture

A biologically active rhodamine conjugate of thyrotropin binds at 4 degrees C to diffusely distributed membrane thyrotropin receptors which patch and become endocytosed into thyroid cells in a temperature-sensitive process. When the cells are first incubated with 8-bromo-cyclic adenosine monophosphate at 37 degrees C, the conjugate also binds to clustered receptors at 4 degrees C. Furthermore, 8-bromo-cyclic adenosine monophosphate reduces the amount of adenosine 3',5'-monophosphate (cyclic AMP) induced by thyrotropin. Hence, increased intracellular cyclic AMP induces receptor patching and reduces the concentration of cyclic AMP normally induced by thyrotropin. This suggests that cyclic AMP acts both as the second messenger of thyrotropin and also as the regulator of the level of thyrotropin receptors.

Localization of 3',5'-cyclic adenosine monophosphate phosphodiesterase (cAMP-PDEase) activity in isolated bovine thyroid plasma membranes

Isolated bovine thyroid plasma membrane preparations were obtained by isopycnic density gradient centrifugation. Cyclic AMP-PDEase (EC 3.1.4.c) activity has been demonstrated by electron microscopic histochemistry on the unit membrane of isolated bovine thyroid cells. 3-isobutyl-1-methyl-xanthine (IBMX) produced partial inhibition, while omission of the substrate revealed no reaction product deposition. These observations correlated well with biochemical studies that showed 0.4% of the total cAMP-PDEase activity to be present in the plasma membrane preparations. Kinetic analysis of cAMP hydrolysis yielded two apparent Michaelis constants for the homogenate and the plasma membrane-rich fraction. Dose-response curves for IBMX inhbition showed cAMP-PDEase of the homogenate to be more sensitive to inhibition than that of the plasma membrane-bound enzyme. Furthermore, wash experiments indicate that the plasma membrane-associated enzyme is tightly bound. This investigation strengthens our previous study and suggests that bovine thyroid cell plasma membranes contain a cAMP-PDEase that may be involved in interactions between the cell and the external environment in a manner yet to be determined.

Chemical composition of porcine thyroid cell plasma membranes

Plasma membranes were isolated from thyroid cells obtained by trypsinization of porcine glands and maintained in culture conditions in the presence or absence of thyrotropin or dibutyryl cyclic AMP. The protein, phospholipid, cholesterol and sialic acid content of the 3 types of cell plasma membranes were very similar. High cholesterol and sialic acid content characterized these membranes. The amino acid and carbohydrate composition was similar to that shown for other eukaryotic plasma membranes. Sodium dodecylsulfate-polyacrylamide gel electrophoresis disclosed the presence of more than 20 protein bands, of which six corresponded to glycoproteins.